LU-2192 tests: enable large xattr testing on zfs

LU-2192 tests: enable large xattr testing on zfs

Since zfs supports large xattr by default and dumpe2fs does not
work on zfs, we need improve large_xattr_enabled() to check the
backend fs type on MDS.

Test-Parameters:mdsfilesystemtype=zfs mdtfilesystemtype=zfs ostfilesystemtype=zfs testlist=sanity,conf-sanity,replay-single,lustre-rsync-test
Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: I27bdad38d8a40e3858d96ee29f44a6abb5365fef
Reviewed-on: http://review.whamcloud.com/4493
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>